For wireless communication, far field transmission and reception of electromagnetic waves is familiar, even to consumers. Applications utilizing near field energy also exist and are becoming increasingly common.
For instance, Radio Frequency Identification (RFID) applications usually employ near field transmitters and detectors. Structures used to transmit and receive near field energy are referred to as Radio Frequency (RF) coils (as opposed to antennas), and include structures such as loop coils, spiral coils, and dipole-like wires, etc. Thus, whereas the term “antenna” refers to a far field radiating structure, RF coils are understood to be for near field applications.
An outgrowth of RFID is Near Field Communications (NFC), which is an extension of the ISO 14443 standard. An NFC device typically includes an interface of a contactless smartcard and a reader and can communicate with smartcards, readers, and other NFC devices. Examples of NFC devices include phones and other handheld devices for applications like wireless payment. NFC communications, at least in the United States, use the ISM band at 13.56 MHz.
Another near field application is Magnetic Resonance Imagine (MRI). MRI machines often include RF coils to produce and/or detect the magnetic field energy that is used to image organs and structures within the body. MRI devices typically operate at about 42.58 MHz/Tesla. Therefore, operating frequency bands will be in the 63.87 MHz band and in the 127.74 MHz band for 1.5 Tesla and 3 Tesla magnetic field systems, respectively.
FIG. 13 is an illustration of conventional near field application 1300. Near field application 1300 includes RF coil 1301, which is an N-turn coil (where N is a positive integer), matching or tuning circuit 1302, transmission line 1303, and RF circuitry (e.g., a transceiver) 1305. RF circuitry 1305 receives and transmits RF signal 1304 over transmission line 1303 to circuit 1302 and RF coil 1301. Conventional RF coils typically employ an external LC circuit as a matching or tuning circuit. Furthermore, conventional RF coils use a continuous conductor so that there is an unbroken current path between the + and − coil terminals.